Thanatephorus cucumeris was long known as Rhizoctonia solani, the vegetative stage. It was long believed to be a sterile fungus, but recently it was discovered that the fungus produces basidiospores. These spores don’t play a role in the epidemics or dispersal of the fungus, only the mycelium is important in this context. Thanatephorus cucumeris is a soil-inhabiting fungus, that is very persistent in the soil. It overwinters as mycelium or sclerotia in the soil and on crop residues. It is also seed-borne. Thanatephorus cucumeris causes diseases such as phytophthora root rot and black leg.
Life cycle and appearance of Thanatephorus cucumeris
The mycelium infects the plant through wounds or the stomata but also directly through the cuticle after the formation of an infection cushion. Dispersal is by rain, water, machinery and tools and in soil particles and via movement of plant parts. The fungus grows from plant to plant and from the soil upward, therefore the lowest plant parts are infected first. Plants that are generally growing well are less susceptible. Optimum temperature for infection is between 15 and 18 °C, but infection still occurs at 35 °C. Disease is most severe in a moderately wet soil.
On the infected plant parts, mycelium and new sclerotia are visible. They fall off the plant onto the soil and thus increase the pathogen density in the soil for the next crop. The mycelium grows in a typical way, with branching at 90 degree angles. This feature makes the fungus more easily identifiable under a microscope.
Within the fungus, 12 different ‘anastomosis groups (AG’s)’ are distinguished. When two hyphae of Thanatephorus cucumeris meet, two different reactions are possible. Hyphae from the same AG fuse. When hyphae from different AG’s meet, the cells around the fusion site die. To complicate matters further, subgroups can be distinguished within AG’s. The different AG’s are more or less specific to the pathogenicity of different host groups.
Not all isolates are pathogenic, many isolates are purely saprophytic soil inhabitants.
The symptoms caused by Thanatephorus cucumeris depend on the time of infection. In general, the fungus causes symptoms on all plant parts that are in or close to the soil. Early infection of the growing tips of the roots shortly after germination causes death of seedlings (‘damping-off’), both before and after emergence. A typical symptom of root infection is that the taproot is reduced to a thread. Reddish-brown lesions occur on the root just below the soil surface. A white collar of fungal mycelium occurs at the base of the stem. This crown rot may result from root infection or from direct infection of the foot. The fungus may also infect plant parts in contact with the soil and from there can grow upward. Lesions are brown and rot on fruits is brown, water-soaked and sunken. Late infection causes top leaves to curl up. The fruits stay firm. The disease may also show up in storage. Potato tubers get a crusty layer (black scab). Symptoms may differ between crops. On the diseased areas, sclerotia can be seen, but they vary in size and colour and are therefore not a good feature to determine the cause of the disease.